Then...who knows? I don't think even the PM knows.

because the backstop doesn't have to happen - if their sacred Malthouse Compromise can work, then they can put it forward after March 29th as the solution which avoids a hard border. But I think they rejected the Draft because they don't themselves believe the Malthouse thing would actually work in practice, and they can't have that exposed when it would mean a customs union instead. So they're insisting on the alternative to Malthouse being 'no deal', because they're OK with that, don't care about a hard border, and want to be anti-EU above all.

Judging by some of the comments at the Guardian live page, she'll peel a few Tories off, but I'd guess not enough, if all the opposition parties vote against this. I think the Commons will then vote to refuse 'no deal', because none of the opposition want 'no deal', and neither do a significant number of Tory Remainers. And I suppose an "extend Article 50" vote will win on the same grounds. But what anyone can do with extra time, god knows (I think the EU would accept extra time, because it's the only way of avoiding 'no deal' after that).

one possible eventual consequence of a bad or non-deal?

It focused on 1020 voters in England. Its findings on Scotland:



On Ireland:



On Gibraltar:



https://wingsoverscotland.com/nations-overboard/

Scottish attitudes toward the question of the Irish backstop and implications for Scotland having a hard border showed overall:



Here's the breakdown by party:



https://wingsoverscotland.com/the-divided-kingdom/

The results are from polls sponsored by pro-Scottish independence blog Wings Over Scotland. Wings runs Panelbase polls on a number of issues from time to time. The results are considered valid enough to be cited by mainstream media when it suits them. But it's just one poll, nearly a year ago.

Have residents in Scotland also been polled as to whether or not they'd choose, if given a choice, to remain under the (unified) British Crown if leaving the UK?

However, during the 2014 Scottish Independence Referendum, Nicola Sturgeon's line was that the Queen would remain as head of state (possibly similar to the situation with countries such as Canada and Australia, though details such as whether there'd be a governor generalship were scant as the issue wasn't really debated).

I suspect it wasn't a fight worth picking given the range of other controversial issues at the time.

I think attitudes in Scotland (indeed, in the UK as a whole, and perhaps the Commonwealth) toward the royal family and the role of the Crown will change a good deal once Elizabeth is no longer queen. And after independence, no Scottish government would be bound by anything that wasn't in a new Scottish constitution anyway, and even if it was in the original version, it could later be amended (hopefully not by referendum!).